The present invention relates to wireless digital communication systems, and more particularly, to communication stations employing code-division multiple access (CDMA) technology utilizing measurement techniques to efficiently determine downlink resource allocation.
In modern wireless communication systems, as the makeup of communication traffic has shifted from primarily voice traffic to an ever-increasing share of data traffic, such as for internet applications, the capacity requirements of such systems have increased. Thus, the provision of techniques to maximize the capacity of downlink (DL) transmissions is highly desirable.
The propagation loss between a transmitter and a receiver is not fixed or constant. In addition to the dependence of propagation loss on distance, variations are caused by obstructions to the path (or multiple paths) between the transmitter and receiver as well as the interaction between paths. These variations are referred to as fading. Additionally, the fading varies with time.
In some communication systems, it is customary to transmit at each time instance to a particular user, or several users among multiple users, who enjoy the most favorable transmission conditions at that time. With these systems, it is necessary to define a channel quality that may be estimated for each user from time to time in order to transmit to each user at the most appropriate moment. Although selection of the most appropriate moment from the fading point of view is not mandatory, instantaneous path loss should be one of the considered factors in the selection.
One measure of channel quality is the instantaneous path loss. Channel quality improves as the instantaneous path loss is reduced, and channel quality is best when the instantaneous path loss is the smallest.
Another measure of channel quality is the interference seen by the user, since higher interference generally requires higher transmission power. As transmission power is limited, it results in reduction of system capacity. Channel quality (CQ) may therefore be defined as the ratio of the received power of a fixed-level base station transmission to the received interference. This ratio is inversely proportional to the required transmission power of the base station for user data. Maximization of this ratio, by continually selecting the users whose CQ is highest (and therefore path loss and/or interference is lowest) at any instant in time, tends to increase system capacity as a whole over time.
The particular signal that is measured to determine the path loss and calculate the ratio is not critical. For example, the signal may be any pilot signal, beacon or even data-carrying signal that is transmitted at a constant or known power. In some systems the reception power is termed received signal code power (RSCP) and the received interference power is termed interference signal code power (ISCP). For example, in the Universal Mobile Telecommunication Systems (UMTS) frequency division duplex (FDD) standard, the common pilot channel (CPiCH) is measured, and the CQ is defined as CPiCH_RSCP/ISCP. In the UMTS time division duplex (TDD) standard, the beacon channel (PCCPCH) is measured and the CQ is defined as PCCPCH_RSCP/ISCP. Since channel conditions change rapidly, it is preferable to use a short time allocation (i.e., a small timeslot) for each transmission. The measurement information used for the allocation must therefore also be timely.
In some communication systems it is customary to separate transmissions to users by time, or to separate one type of user-selective transmission in time from other types of transmissions, such as normal voice services and data services. Such time separation can be obtained in different ways. For example, a repetitive frame may be divided into a plurality of timeslots. Each timeslot may each be allocated to one or more users at a time. In addition, several timeslots, adjacent or non-adjacent, may be allocated to one or more users. If a collection of one or more timeslots is allocated together, it may be referred to as a sub-channel.
In a time-separated transmission, it is likely that the interference in all of the timeslots or sub-channels is not equal. The reporting of a single value for all timeslots often results in a non-optimal allocation and the information in some of the timeslots may be lost. It is therefore desirable to report individual measurements for each timeslot.